Liquid chromatography is widely used as a technique for analyzing sample components. More specifically, liquid chromatography is a separation technique utilizing a difference in the distribution of substances between two phases, i.e., between a stationary phase formed on the surface of a column packing material or on the inner surface of a capillary column and a mobile phase flowing through gaps in the packing material or the capillary column. Components of a sample pass through the column at different rates according to their respective chemical properties.
In a case where two or more substances having very different chemical properties from each other are analyzed all together, there is a case where it takes much time to analyze them. Therefore, in order to reduce analysis time while keeping a separation pattern, a mobile phase composition gradient elution method, in which the composition of a mobile phase is changed with time, is often used. The mobile phase composition gradient elution method is based on the fact that the distribution of substances is changed by changing the composition of a mobile phase.
FIG. 4 is a diagram showing a liquid chromatograph for mobile phase composition gradient elution.
As shown in FIG. 4, the liquid chromatograph includes mobile phase flow channels 2a and 2b for sending mobile phases 1a and 1b and liquid sending pumps 5a and 5b provided in the mobile phase flow channels 2a and 2b, respectively. The flow rate of each of the liquid sending pumps 5a and 5b is regulated by controlling the number of revolutions of motors. The mobile phase flow channels 2a and 2b join together at a mixer 11 where the mobile phases 1a and 1b are mixed and then flown into an analysis flow channel 12. The analysis flow channel 12 has a sample injection portion 7, a separation column 15 provided downstream of the sample injection portion 7 and a detector 19 provided downstream of the column 15.
A sample injected into the analysis flow channel 12 through the sample injection portion 7 is introduced into the separation column 15 by a mobile phase, obtained by mixing the mobile phases 1a and 1b by the mixer 11, to be separated into its components and then detected by the detector 19.
The flow rate of each of the liquid sending pumps 5a and 5b is controlled by a control unit 21 so as to be changed according to a predetermined mobile phase sending program to accomplish gradient elution.
In the case of using such a liquid chromatograph, as shown in FIG. 5, analysis is carried out by, for example, changing the sample retention power of the column 15 by gradually changing the composition of a mobile phase in such a manner that the concentration of a mobile phase A is decreased from 100% (initial) to 0% (final) and the concentration of a mobile phase B is increased from 0% (initial) to 100% (final). Particularly, such a gradient system, in which two or more liquid sending pumps are used to mix two or more mobile phases at a position downstream of the pumps, is called a “high-pressure gradient system” (see, for example, Patent Document 1). It is to be noted that in FIG. 5, the coordinate A on the vertical axis represents the concentration of the mobile phase A being 100%, the coordinate B on the vertical axis represents the concentration of the mobile phase B being 100%, and the horizontal axis represents time.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-98166    Non-Patent Document 1: Hideko Kanazawa, “Temperature-Responsive Chromatography”, BUNSEKI, pp. 303-308, vol. 6, 2004